Before trying to sort this out, I need to understand the configuration.
Typically tandems are linked with a chain loop and must turn together. Then, usually, the rear crank drives the bike.
Your description seems to imply that this is not the case here, and some kind of crank allows one ,(front?) crank to idle while the other drives.
This seems odd because it allows the riders to be out of phase with each other, which can be problematic.
So, please better describe the system, and which crank directly powers the bike.
Last edited by FBinNY; 03-10-24 at 05:02 PM.